It is known that in order to achieve high speeds in boats, it is necessary to lift the hull out of the water so as to reduce friction and wave-impact. This is achieved by entrusting the two functions of propulsion and lift to separate systems; propulsion is achieved by underwater screw propellers or airscrews and lift is achieved by inclined bottoms, submerged fins, or air pressure under the hull (an air cushion). The boats are generally steered by rudders.